1. Field of the Invention
The present invention relates generally to a photometric apparatus and, more particularly, to an improved photometric apparatus having plural light sensors for detecting light from different regions at plural luminance measurement points and having means for enhancing accuracy of photometry by increasing the number of luminance measurement points in regions where the respective light sensors have a non-linear characteristic.
2. Description of the Related Art
Conventionally, a photometric apparatus used, for example, as an exposure meter for a camera comprises an incident light measuring unit for measuring an average brightness of light irradiating a subject to be photographed, and a reflected light measuring unit for measuring brightness of a particular portion of the subject.
FIG. 4 shows an example of the above-described photometric apparatus. In the illustrated apparatus, incident light is measured using a white ball 102 arranged on a front surface of a body case 101. In connection with the measurement of incident light, light is input from a first area corresponding to a relatively large angle of visibility (for example, 30xc2x0 to 40xc2x0) through the white ball 102 arranged on the front surface of the body case 101 and is measured using a first light sensor SPD1, such as a silicon diode or the like, arranged behind the white ball 102 to detect the received light. In response to the output of the first light sensor SPD1, the average brightness of the surrounding area is measured. That is, brightness of the first area is measured on the basis of an output of the first light sensor SPD1. Measurement of reflected light is performed by so-called spot photometry, which uses a lens 103 arranged on a rear surface of the body case 101 to receive light from a second area corresponding to a relatively small angle of visibility (for example, 5xc2x0 to 10xc2x0). For this purpose, a second light sensor SPD2, such as a silicon diode or the like, is arranged behind the lens 103 to detect the light received from the second area. In response to the received light, the brightness of a particular portion of the first area, that is, the brightness of the second area, is measured on the basis of output of the second light sensor SPD2.
In many cases, photometric devices are provided with a viewfinder 104 which allows the user to visually define the specific portion of the subject for which measurement of reflected light is desired. Normally, a display unit 105 for displaying the results of measurement and a mode switch SWm are also provided on the front surface of the body case 101 where the white ball 102 is provided. The lens 103 may be mounted on the surface of the body case 101 opposite the surface on which the white ball 102 is provided, or the lens may be mounted so that it is rotatable relative to the position of the white ball 102 on the body case 101.
As described above, the second light sensor SPD2 used for measurement of reflected light receives light collected from a smaller area than that used for measurement of incident light. Therefore, when performing measurement of incident light and measurement of reflected light under the same ambient luminance, the quantity of light incident upon the second light sensor SPD2 used for measurement of reflected light is smaller than the quantity of light incident upon the first light sensor SPD1 used for measurement of incident light. Thus, the photoelectric current output by the second light sensor SPD2 is also smaller than that output by the first light sensor SPD1. As is often the case, identical components are used in parallel light detection circuits to simplify construction and avoid errors due to circuit constants occurring when circuit components of different construction are used. Accordingly, in the case where light sources having the same structure are employed for the first and second light sensors SPD1 and SPD2, and photoelectric current-to-voltage conversion circuits having the same performance characteristics are used to convert photoelectric currents output by the light sensors into voltages, the relationship between input and output becomes that as shown in FIG. 5. More specifically, when photometry is performed under the same ambient luminance, the output of the photoelectric current-to-voltage conversion circuit associated with the second light sensor SPD2 used for the measurement of reflected light has a smaller value than that output by the photoelectric current-to-voltage conversion circuit associated with the first light sensor SPD1 used for the measurement of incident light.
An input/output characteristic of the light sensors will be described below with reference to FIG. 5. Generally, the relationship between the input (luminance value LV) versus the output (exposure value EV) characteristic of a light sensor used in exposure meters containing a photoelectric current-to-voltage conversion circuit is linear in a predetermined range of luminance values, but tends to become non-linear in its output in areas having higher or lower luminance values than the predetermined range of luminance values. This loss of linearity is usually more pronounced on the low luminance side of the predetermined range. Such tendency is illustrated in the graph shown in FIG. 5. Due to the above-described difference in the quantity of light incident upon the first and second light sensors SPD1 and SPD2, the photoelectric current output by the second light sensor SPD2 begins on the low luminance side to become non-linear at a higher luminance value (LV about 6) than that of the first light sensor SPD1. Conversely, because a higher intensity light is incident on the first light sensor SPD1, the photoelectric current output by the first light sensor SPD1 begins on the high luminance side to become non-linear at a lower luminance value (LV about 15) than that of the second light sensor SPD2. As a result, the predetermined range of luminance values having linearity, for which signal processing is easily conducted, is normally deemed the measurable luminance range of the device.
A method of linear interpolation such as that disclosed for example, in Japanese Patent Laid-Open No. 44018/1992 (which is incorporated herein by reference), has been used for correcting errors in measurement results due to errors in products into design values. Simply explained, the method of linear interpolation disclosed therein comprises the storing of design values (design data) and actual measurement values (actual measurement data) of outputs from a plurality of measurement points, which are preset for the ambient luminance, finding design values of outputs for the measurable area on the basis of the preset values stored by way of linear interpolation in actual use, and correcting actual measurement values in actual use into the design values thus found by linear interpolation, on the basis of characteristic errors between the stored actual measurement values and the design values. For example, with a photometric apparatus having one sensor, photometry is actually performed in brightness at a plurality of predetermined luminance measurement points in a process of manufacture or product inspection, and actual measurement values and design values at that time are stored as correction data in a nonvolatile memory such as EEPROM or the like. During actual use of the device, the correction data stored in nonvolatile memory is used to conform actual measurement values obtained by the photometric apparatus to design values obtained by performing linear interpolation on a plurality of design values in the correction data stored in the nonvolatile memory.
While an example of linear interpolation using one sensor is described above, a method of correcting actual measurement values in actual use into design values is carried out under the same general scheme as described above in the case where two sensors are used. For example, photometry is actually performed in brightness (LV=1, 4, 7, 10, 13, 16 in the case shown in FIG. 6) at ambient luminance measurement points, respectively, as indicated by P51 to P56 and Q51 to Q56 during manufacture and product inspection. Actual measurement data and design data, which are obtained by the first and second light sensors SPD1, SPD2 at the respective ambient luminance measurement points, are stored as correction data in the nonvolatile memory. Actual measurement values obtained by the first and second light sensors SPD1, SPD2 are corrected into design values during actual use of the photometric device by the use of the correction data stored in EEPROM.
Conventionally, as shown in FIG. 6, a plurality of preset ambient luminance measurement points used for photometry with a plurality of light sensors, such as the fist and second light sensors SPD1 and SPD2, are the same in number and luminance.
As described above, a plurality of sensors SPD1 and SPD2 having different quantities of incident light even in the same ambient luminance are different in input/output characteristics on low and high luminance sides in a predetermined luminance area of the ambient luminance, resulting in the drawback that one sensor has linear input/output characteristics while another sensor does not have linear input/output characteristics.
This drawback presents a problem where preset luminance measurement points are the same in number and luminance value. In such circumstances, when it is attempted to conform design values by increasing the number of ambient luminance measurement points in one light sensor so as to improve its output accuracy in a given luminance area, a large amount of unnecessary data is produced in areas where input/output characteristics are linear. For example, when an area which presents a non-linear, curvilinear input/output characteristic as illustrated by a broken line in FIG. 7 is selected for correction using linear interpolation, to produce actual, curvilinear design values with high accuracy, there is a disadvantage in that measurements using a light sensor which has a linear input/output characteristic in that area will produce a large amount of unnecessary correction data. Thus, the amount of unnecessary data becomes large. On the other hand, when the number of ambient luminance measurement points in the luminance area is reduced to eliminate such a disadvantage, a difference between the design values thus obtained by linear interpolation and the actual, curvilinear design values becomes large on a side having non-linear input/output characteristics to thereby degrade the accuracy of output values.
In view of the foregoing drawbacks of the conventional art, an object of the present invention is to provide a photometric apparatus capable of obtaining accurate luminance measurements using a plurality of light sensors having the same characteristics.
Another object of the present invention is to provide a photometric apparatus having means for enhancing accuracy of photometry by performing linear interpolation without producing a large amount of unnecessary data.
Still another object of the present invention is to provide a photometric apparatus of the foregoing type in which the accuracy of photometry is enhanced by separately setting the number of respective ambient luminance measurement points for a plurality of light sensors in accordance with the input/output characteristics of the respective light sensors.
Yet another object of the present invention is to provide a photometric apparatus of the foregoing type in which the accuracy of photometry is enhanced by increasing the set number of luminance measurement points used where respective photosensors have non-linear input/output characteristics.
In order to achieve the foregoing objects and others, in one aspect of the present invention a photometric apparatus is provided with improved accuracy. The photometric apparatus comprises a first light sensor for receiving light from a first area, a second light sensor for receiving light from a second area smaller than the first area, a brightness detecting circuit for outputting a first actual measurement value representing brightness in the first area in accordance with an output of the first light sensor and outputting a second actual measurement value representing brightness in the second area in accordance with an output of the second light sensor, a storage circuit for storing actual measurement data and design value data of the first light sensor obtained at a plurality of first ambient luminance measurement points and actual measurement data and design value data of the second light sensor obtained at a plurality of second ambient luminance measurement points, and a correction circuit for correcting the first and second actual measurement values, respectively, into first and second design values, based on the actual measurement data and design value stored, wherein the number of second ambient luminance measurement points is set to be larger at low ambient luminance values than the number of first ambient luminance measurement points and/or the number of first ambient luminance measurement points is set to be larger at high ambient luminance values than the number of second ambient luminance measurement points.
By the above construction, photometry with enhanced accuracy is made possible because the number of respective ambient luminance measurement points for the first and second light sensors may be set separately in accordance with the input/output characteristics of the first and second light sensors. Accordingly, the plurality of second ambient luminance measurement points is set to be larger or more dense at low ambient luminance values than the number of first ambient luminance measurement points. Hence, it is possible to enhance accuracy in photometry without an increase in unnecessary actual measurement data and design value data in the case where an output of the brightness detecting circuit based on an output of the second light sensor is non-linear and an output of the brightness detecting circuit based on an output of the first light sensor is linear. In particular, it is possible to enhance accuracy of photometry measurements on a low ambient luminance side where a minute photoelectric current is treated, measurement points vary greatly due to noise, and the quantity of incident light is small.
Also, it is possible to enhance accuracy of photometry measurements without an increase in unnecessary actual measurement data and design value data in the case where an output of the brightness detecting circuit based on an output of the first light sensor is non-linear and an output of the brightness detecting circuit based on an output of the second light sensor is linear because the number of first ambient luminance measurement points is set to be larger or more dense at high ambient luminance values than the number of second ambient luminance measurement points.
By providing that the number of second ambient luminance measurement points is set to be larger in number than the number of first ambient luminance measurement points, it is possible to enhance accuracy of photometry measurements without an increase in unnecessary actual measurement data and design value data in the case where an output of the brightness detecting circuit based on an output of the second light sensor is non-linear and an output of the brightness detecting circuit based on an output of the first light sensor is linear.
Also, by providing that the number of first ambient luminance measurement points is set to be larger in number than the number of second ambient luminance measurement points, it is possible to enhance accuracy of photometry measurements without an increase in unnecessary actual measurement data and design value data in the case where an output of the brightness detecting circuit based on an output of the first light sensor is non-linear and an output of the brightness detecting circuit based on an output of the second light sensor is linear.